1. Field of the Invention
This invention relates to lead storage batteries and more particularly, to a method for manufacturing a novel and improved electrode for use in the batteries.
2. Description of the Prior Art
The charge and discharge cycle life of lead storage batteries is, in most cases, related to the bonding force of active materials per se in both positive and negative electrodes. Especially with a positive electrode, the particles of active material are apt to be broken into pieces during the course of repeated charge and discharge operations with the attendant drawbacks that the particle-to-particle bond is lowered, the particles are softened and fall off, and the fallen pieces deposit on various portions of the battery to cause short-circuit problems. Thus, the life cycle of the positive electrode is chiefly attributed to the bonding force between active material particles not only in the paste electrodes but also in the clad-type or tubular electrodes. On the other hand, the negative electrode suffers, aside from degradation owing to the contraction of active materials, from the disadvantage that the bonding force of active material particles becomes weakened in portions of the plate to which pressure is minimally applied.
In order to overcome the above drawbacks, it is usual to use binders: the electrodes are impregnated with dispersions of polyethylene, fluorocarbon polymers such as polytetrafluoroethylene (U.S. Pat. No. 3,630,781) and similar synthetic resins, by which active material particles are bonded together using these binder resins. However, imparting sufficient binding strength requires an increasing amount of resin, which impedes diffusion of electrolyte through the porous body, resulting in a considerable increase in resistance of the battery. Even though active materials or starting materials for the active materials are mixed with these resins to improve the binding force, the resins become fibrous or form a network structure in the inside of the particulate active materials. This is useful in binding large-size secondary particles together but cannot protect finely divided particles of active material. The network structure which has once been formed may be broken during the mixing, which involving difficulty in controlling the mixing conditions. For the protection of active materials of an electrode for use in lead storage batteries, it is ideal that while finely divided active material particles are protected, the protective structure is formed as porous, finally producing the binding force between secondary particles. Accordingly, there has been a demand for materials or methods for making protected electrode which meet the above requirements.